The concept of Satan as being an evil entity is one that evolved over many centuries. In the Hebrew Bible, or Tanakh, and in the Christian Old Testament, Satan is rarely mentioned. Where Satan does appear, he is clearly a member of God's court and plays the role of the Accuser (possibly one of a number), much like a prosecuting attorney for God. The chief example of this occurs in the Book of Job, where Satan seeks God's permission to persecute God's servant Job. However, Job survives the ordeals that Satan imposes upon him, proves himself to be a moral and upright man in the eyes of God, and is restored to his former state. Another reference to Satan occurs at I Chronicles, which reports that Satan moved King David to take a census; a parallel passage in II Samuel reports that it was God who was responsible. (2 Sm 24.1: "And again the anger of the LORD was kindled against Israel, and he moved David against them to say, Go, number Israel and Judah," 1 Chr 21.1: "And Satan stood up against Israel, and provoked David to number Israel," both KJV.)
Satan figures much more prominently in the New Testament and in Christian theology generally. As reflected in John Milton's epic poem Paradise Lost, Satan is believed to have been an archangel named Lucifer who turned against God before the creation of man. (Prophesies in Isaiah 14 and Ezekiel 28 are sometimes thought to be referring to Satan, rather than to the earthly king that a plain or literal reading of the text suggests.) According to this view, Satan waged war against God, his creator, and was banished from Heaven because of this.
The creation story found in the book of Genesis reports that a serpent tempted Adam and Eve to partake of the fruit of the Tree of Knowledge. In the Jewish tradition, the serpent was always taken to be literally a snake; the story tells us the origin of how the snake lost its legs. Later Christian theologies interpreted this serpent to be Satan, to the point where many American Christians are unaware that the actual Hebrew text does not identify the serpent as Satan. In Eastern Orthodox Christianity, Satan is one of humanity's three enemies, along with sin and death.
According to most Christian eschatology, Satan will wage a final war against Jesus, before being cast into Hell for all eternity. The Unification Church teaches that Satan will be restored in the last days and become a good angel again (see Lucifer, A Criminal Against Humanity (http://www.tparents.org/library/unification/books/lcah/0-toc.htm)). A few early Church Fathers are known to have prayed for Satan's eventual repentance, it was not generally believed that this would happen.
In various Gnostic sects, Satan was praised as the giver of knowledge, sometimes with references to Lucifer, "the light-bringer." Some claimed that the being imagined as God by Christians and Jews was in fact Satan, as a world as imperfect as ours could not be created by a perfect God.
Particularly in the Medieval period, Satan was often depicted as having horns and a goat's hindquarters. He has also been depicted as carrying a pitchfork, and with a forked tail. None of these images seem to be based on Biblical materials. Rather, this image is apparently based on the Horned God, specifically Pan/Dionysus, common to many western mythologies. Whether or not the horned Satan was created intentionally to discredit the God of other religions is unknown, but it has been alleged.
There are historical records of people worshiping Satan, though their authenticity is sometimes questioned. Today, some people identify themselves as Satanists. Of these, some claim that Satan is a real being, some view him as a symbol for the animal desires of humans, and some view him as a symbol for the rebellious or independent aspects of humanity. Some Christians believe that certain other religions are satanic, that is, influenced by and supported by the power of Satan.
Paganism is one of the religions most often seen as satanic by some Christians (see Jack Chick). However, this is a minority view and one not held by most mainstream Christians, and Neopagans are somewhat sensitive to these accusations. While Neopagans often include deities of other religions in their practice, they almost never include Satan.
Some individuals identifying themselves with the New Age thought process believe that Satan, or Lucifer, was the leader of extraterrestrials who came to Earth and waged a galactic war with another extraterrestrial group lead by one now referred to as "God". This is not necessarily the belief of those standing behind that system of thought.
Accuser - Satan is styled the "accuser of the brethren" (Rev. 12:10. Comp. Job 1:6; Zech. 3:1), as seeking to uphold his influence among men by bringing false charges against Christians, with the view of weakening their influence and injuring the cause with which they are identified. He was regarded by the Jews as the accuser of men before God, laying to their charge the violations of the law of which they were guilty, and demanding their punishment. The same Greek word, rendered "accuser," is found in John 8:10 (but omitted in the Revised Version); Acts 23:30, 35; 24:8; 25:16, 18, in all of which places it is used of one who brings a charge against another.